The progression of stanzas in "She Walks in Beauty" tell of various aspects of the beauty of a woman. The first stanza compares the woman to the beauty of the night

She walks in beauty, like the nightOf cloudless climes and starry skies;And all that’s best of dark and brightMeet in her aspect and her eyes:Thus mellowed to that tender light 5Which heaven to gaudy day denies.

Describing her as walking "in beauty" makes her beauty less personal and more ethereal. Since she is compared to something as cosmic as "night of cloudless climes and starry skies" -- that is, a clear night lit brightly by stars -- the suggestion is not just of personal beauty, but of a celestial, almost spiritual quality. It is not simply that she is dark-haired, with bright eyes (like the dark sky of night, and bright stars,) but "all that's best of dark and bright" are joined in her aspect (looks) and her eyes. The picture, created in just these first six lines, is of a woman who is not only blessed with physical beauty, but has a certain quality of harmonious nature about her which increases her attractiveness. A phrase so general as "all that's best of dark and bright" brings to mind the beauty of all things dark and things shining. To be possessed of the best qualities of beauty of such a large class of things is substantial indeed.

One shade the more, one ray the less,Had half impaired the nameless graceWhich waves in every raven tress,Or softly lightens o’er her face; 10Where thoughts serenely sweet expressHow pure, how dear, their dwelling-place.

In this stanza, Byron talks about how not only is the subject of the poem beautiful, but she has a perfect balance to her beauty. "One shade the more, one ray the less" refers back to the first stanza's mention of dark and bright. She has the right amount of "shade" (darkness) and "rays" (brightness) balanced in the look of her face. But for the first time Byron now discusses her thoughts illuminating her beauty. "Where thoughts serenely sweet express" means that the thoughts of her mind are so sweet that they make her countenance so. The thoughts, he is saying, express that their "dwelling place" i.e. -- her head, is pure and dear. This is a markedly different kind of personification, in which a person's thoughts are being said to have an expression about the body they inhabit.

And on that cheek, and o’er that brow,So soft, so calm, so eloquent,The smiles that win, the tints that glow, 15But tell of days in goodness spent,A mind at peace with all below,A heart whose love is innocent!

Byron is still talking about her looks, but in this stanza he is focusing on what her looks say about her soul. The first four lines are telling of her face, which has "smiles that win, the tints that glow" but everything is meant to show that her days are "in goodness spent". What has started out as a poem about how a woman looks has come to be about how her looks illuminate what kind of person she is. The fact that she looks a certain way means, to Byron, that she is good and kind and has a "heart whose love is innocent". Byron feels so strongly about this last line that he adds an exclamation point. The effect is that while the poem may have seemed to be one about superficial beauty, what the poet really admires is the subject's goodness.